please I need help identifying my transmission

[pelusayco]

i bought a 86 chevy astro cargo van with 2.5 4 cyl on line, the van came with a saginaw 4 speed, but the second speed gear is stuck , and i can't take it off, now i need to rebuild it, or with which transmission can I replace it?,
the numbers on it are:

case extension (tailshaft) 14076802 or 14078802

side cover GM 6261888

transmission case 1407 5800
NFc8
GM 5225
R6 R21

bearing retainer 3859033


and on the main drive gear has one grove
please help me